The present invention relates generally to mold machining, and in particular to a method and apparatus for forming a mold for golf club grips.
There are approximately 26.4 million golfers in the United States. In 1999, these golfers spent $5.9 billion on equipment, $2.5 billion of which was spent on golf clubs alone. One component of this significant market is the rubber grip that covers the end of each golf club. In addition to the grip attached to each new golf club sold, a large independent golf club grip market exists, as many golfers have the grips on their golf clubs replaced periodically.
A typical golf club grip is cast from rubber in a mold. It is hollow, to fit over the end of the golf club, with a constant internal diameter, typically about 0.580 or 0.600 inches. The golf club grip is generally cylindrical in overall shape, tapering from a larger diameter at the end, or cap, to a diameter only slightly greater than that of the golf club shaft at the mouth.
A variety of designs may be formed into the surface of the golf club grip as indentations, or voids. These designs are typically cast into place by operation of a corresponding raised or xe2x80x9cnegativexe2x80x9d image of the design on the surface of the golf club cavity. These designs usually include the name and/or logo of the golf club manufacturer, a brand name identifying the model or line of golf club, and other identifying information. Additionally, the bulk of the surface area of the golf club grip may contain an array of texture featuresxe2x80x94curves, crosses, stars, chevrons, and the likexe2x80x94formed therein. These patterns provide both aesthetic appeal and facilitate the golfer""s grip on the club. The entire surface design, including all brand names, logos, texture features and other design elements, may additionally be partially or completely filled with paint or other suitable pigment, thus providing a contrasting color to the design.
The molds for casting golf club grips are typically manufactured using a multi-step process resulting in a cast steel mold. Initially, a master golf club grip form, with the desired tapered shape and surface design formed therein, is fashioned from a suitable material, such as aluminum. Due to the shrink ratios and tolerance stacking inherent in the manufacturing process, the master form is proportionally larger than the desired final golf club grip. A mold of rubber or other suitable material is cast around the master form, and subsequently cut into halves, removed and reassembled to form a hollow mold. One of ordinary skill in the art will readily recognize that the surface design from the master form is cast into the rubber mold in a dimensionally negative image, i.e., the design features that exist as indentations or cuts into the surface of the master form, appear as raised features or protrusions in the inner surface of the rubber mold. A ceramic form is then cast in the rubber mold. The ceramic form resembles the original master form, but is proportionally smaller due to shrink ratios inherent in both the rubber and ceramic casting processes. The ceramic form, like the master form, contains the surface design features as indentations formed therein. A steel mold is subsequently cast around the ceramic form. The ceramic form is then broken and removed from the mold, and a plurality of golf club grips may be cast from the resulting steel mold.
While the above-described process is well understood and fairly easy to perform, it has significant drawbacks. As mentioned, each molding and casting entails an inherent shrink ratio by which the mold or cast product is slightly smaller than its parent form or mold. Thus, the dimensions of the desired final productxe2x80x94the golf club gripxe2x80x94must be scaled up to manufacture the master form, a process that is inexact and largely empirical. Additionally, the inherent inaccuracies of the casting process, particularly the steel casting of the final grip mold, results in a finished product wherein the quality of the surface design, i.e., the sharpness of the edges and corners of the surface design features, is degraded. Also, due to compounding registration errors at each casting step, the joints or seams where the various mold halves fit together must be buffed or otherwise reworked to remove joint lines. In severe cases, extensive buffing may be required, which will tend to occlude adjacent surface design features. The process is time consuming, and thus is not ameliorable to quick turn-around for new or different surface designs. The manufacturing process also suffers from a general lack of detail in the finished product surface design features, and a general lack of consistency between casting runs.
Electronic discharge machine (EDM) machining is well known in the metallurgical arts, and has long been recognized as applicable to the cutting of precise shapes, particularly those including embedded surface designs. In a typical mold cavity cutting application, the EDM process comprises cutting the negative image of a desired surface design into an electrode, usually made of graphite. A high electrical voltage is applied to the electrode, a corresponding ground is applied to the mold blank, and the two are separated by a dielectric fluid. As the electrode and mold blank are brought into close proximity (under the control of a numeric positioning unit), the dielectric fluid ionizes and becomes an electrical conductor, allowing a spark of current to flow across the gap. Like an arc welder, this high intensity spark erodes a small amount of material from the mold blank. Through the eroding action of many repeated sparks as the electrode is repositioned with respect to the mold blank, the mold blank is worn away until it assumes the negative of the size, shape, and surface design of the electrode.
EDM machining has heretofore proven unsuitable for machining golf club grip molds, however, due to the complex surface designs that must be reproduced, and particularly due to the taper in the grip""s cylindrical shape. As will be explained below, to cut the surface design into the cavity, the EDM electrode must be of a smaller diameter than the desired cavity, and the electrode moved within the cavity, to impart the design equally across the mold cavity surface. Thus, the surface design as prepared for the golf club grip must be scaled before being formed into the smaller diameter EDM electrode, in such a manner that the dimensions and relative positions of design elements are preserved after the design is subsequently cut into a mold cavity. Typical Computer Aided Design (CAD) software accomplishes such scaling by xe2x80x9cunwrappingxe2x80x9d the 3-dimentional design into a flat, 2-dimentional form, applying a linear scaling factor in one or more dimensions, and then xe2x80x9cwrappingxe2x80x9d the scaled, flat design back to a 3-dimentional form. While this technique may work well for straight cylindrical shapes, it results in distortions when applied to a surface design on a tapered cylindrical shape. Hence, without a method of accurately mapping the golf club grip design image to a tapered electrode, EDM machining is incapable of accurately reproducing a design within a cavity, and thus the mold cannot subsequently produce accurate golf club grips.
The present invention entails a mold for casting golf club grips, the mold comprising a generally cylindrical cavity having at least one tapered portion, with the cavity including a negative image of a golf club grip design formed thereon by an electrical discharge machine.
In another aspect, the present invention relates to a method of transferring a design for a golf club grip comprising a plurality of design elements to an EDM electrode. The method comprises selecting two corresponding size parameters, a first parameter associated with a golf club grip design and a second parameter associated with the electrode, determining the ratio of the first size parameter to the second size parameter, scaling the design elements associated with the golf club grip design by a function of the ratio of size parameters in such a way that their cordal relationship is preserved, and transferring the scaled design to the electrode.